Prior studies of overground manual wheelchair propulsion have utilized instrumented push rims, which measure triaxial forces and moments exerted by the hand on the push rim during the push phase of propulsion. However, instrumented push rims are heavy and are impractical for quantifying manual wheelchair dynamics during daily life. We aim to develop an approach to quantify external forces during wheelchair propulsion using wheelchair-mounted inertial sensors, which are small and lightweight. In this pilot study, we compared the acceleration of a wheelchair (from inertial sensors) to the acceleration of the system's center of mass (combined mass of the wheelchair and user from an instrumented treadmill) during manual propulsion. We found that the acceleration of the wheelchair alone was considerably different than the acceleration of the system's center of mass, suggesting that the acceleration of the upper extremities during propulsion impacts the dynamics of the wheelchair.